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국부적 용융이 ${Si_3}{N_{4p}}$/2124 Al 복합재의 초소성 거동에 미치는 영향 (II)
정하국,김혜성,Jeong, Ha-Guk,Kim, Hye-Seong 한국재료학회 2001 한국재료학회지 Vol.11 No.7
많은 연구결과들은 국부적 용융체의 존재가 고온인장 변형 시 발생하는 내부공극의 발달을 억제할 수 있음을 보고하고 있다. 그러나 이러한 국부적 용융체가 존재한다고 해서 반드시 고변형속도 초소성 현상이 관찰될 수 있는 것은 아니다. 금속기지와 보강재간의 계면에 국부적 용융체의 양이 너무 많이 존재하면 두상간의 결합력이 떨어져 금속기지상으로부터 보강재가 분리되는 현상이 야기될 수 있기 때문이다. 그러므로, $Si_3$$N_{4p}$ 2124 Al 복합재의 초소성 유동 특성을 이해하기 위해 변형온도에 따른 미세구조 변화와 계면특성을 조사하였다. 본 연구를 룽해 $Si_3$$N_{4p}$ 2124 Al 복합재에서 Al-기지와 $Si_3$$N_{4p}$ 강화상간의 계면상의 국부적 용융이 시작되는 온도부근에서는 큰 초소성 특성이 얻어지지만, 국부적 용융이 시작되는 온도를 지난 인장온도범위에서는 오히려 초소성 특성이 현저하게 저하되는 현상이 관찰되었다. 위의 실험결과는 $Si_3$$N_{4p}$ 2124 Al복합재의 고변형속도 초소성 거동에 기여하는 최적의 액상량이 존재한다는 것을 의미한다. Many experimental results have revealed that the development of cavitaition during tensile deformation is limited by the Presence of liquid phases. However, the presence of liquid phases does not always lead to high-strain-rate superplasticity, because too much liquid causes intergranular decohesion at grain boundaries/interfaces in metal-matrix composites. Thus, it is important to examine the nature of interfaces of superplastic composites in order to understand the origin of superplastic flow related to liquid grain boundaries during high-strain-rate superplastic deformation. This study shows that a large elongation is obtained at the temperature, that is close to the onset temperature for partial melting in the superplastic composites, but the elongation significantly decreases at slightly higher temperatures, which are close to the end temperature fur partial melting. This indicates that there is an optimum amount of the liquid phase for obtaining high-strain-rate superplasticity in these materials.
Mg-1.4 wt%Ca-xwt%Zn 합금의 이단 용체화 및 시효처리에 따른 미세구조 변화와 물성에 관한 연구
구성모 ( Seong Mo Koo ),정하국 ( Ha Guk Jeong ),김택수 ( Teak Soo Kim ),김혜성 ( Hye Sung Kim ) 대한금속재료학회(구 대한금속학회) 2015 대한금속·재료학회지 Vol.53 No.9
Optimum heat treatment conditions to improve the hardness and corrosion resistance of ternary Mg-Ca-Zn alloys have been studied, based on the theoretical models and DSC (Differential scanning calorimetry) experimental data. Two-step heating process at 420 ℃ and 480 ℃ has been applied and we have found that the low melting point phase, Ca2Mg6Zn3 can effectively be dissolved into α-Mg matrix without premature melting. Due to preceding treatment at lower temperature followed by the second stage solid solution heat treatment at 480 ℃, Mg-1.4 wt%Ca-xwt%Zn alloys (x=0, 1.5 and 4.0) exhibit improved corrosion resistance than that from the single step solid solution treated alloy at 480 ℃. However, aging treatment of the alloy at 200 ℃ has led to the homogeneous precipitation of Ca2Mg6Zn3 and Mg2Ca phases in the matrix as well as at the grain boundary. This microstructural change results in the deterioration of corrosion resistance mainly originated from galvanic corrosion between the matrix and the precipitates. The hardness of Mg-1.4%Cax% Zn alloy, on the other hand, significantly increases with Zn addition by applying two-step solid solution and aging heat treatment.
Mg-Ca 합금의 전기화학적 특성에 미치는 Ca 첨가량 및 열처리의 영향
이상희 ( S. H. Lee ),정동석 ( Dong Seok Chung ),박병옥 ( B. O. Park ),김용길 ( Y. G. Kim ),정하국 ( Ha Guk Jeong ),김혜성 ( Hye Sung Kim ) 한국열처리공학회 2004 熱處理工學會誌 Vol.17 No.3
N/A In this study, the influence of heat treatment and Ca contents on the electrochemical behavior was investigated. Mg-Ca alloys, i.e., Mg-0.22wt%Ca, Mg-0.56wt%Ca, Mg-1.31wt%Ca are prepared by ingot metallurgy. As-cast Mg-Ca alloys exhibited better electrochemical properties than pure Mg. Especially, Mg-0.22wt%Ca alloy improves its anode efficiency up to 62% and lowers the OCP up to -1.72VSCE. Microstructure and XRD patterns of Mg-Ca alloys show that additive Ca element is mainly solid-solutioned. While, the others show the microstructure and XRD pattern with large Mg₂Ca at grain boundary. To assess the effect of heat treatment on the as-cast Mg-alloy, the specimens were heat treated at 200℃ for 2 hours under CO2 gas atmosphere. Although corrosion properties of Mg-Ca alloys are somewhat deteriorated by heat treatment at 200℃ Mg-0.22wt%Ca alloy with uniformly distributed nano-sized Mg,Ca phase in a-Mg matrix show still better corrosion properties than pure Mg specimen.